Republicans working on the Senate race in Colorado sensed something was shifting even before the votes were counted.
Voters in suburban Denver — who had backed President Obama twice — were abandoning Senator Mark Udall, a Democrat, in the final days before the election.
“We thought the evening could be decided early,” said Cory Gardner, the Republican who defeated Mr. Udall.
More striking than any Republican gains in red-state America on Tuesday were the party’s Senate victories in Colorado and North Carolina and the near miss in Virginia.
All are states that both parties believed were trending Democratic, and that Democrats boasted would before long be out of reach to Republicans.
But a powerful lesson for both parties emerged from the returns: Demographic shifts that are gradually reshaping the American electorate, making it more racially diverse and younger, cannot overcome a difficult political environment and a weak message in a nonpresidential year.
And the Democratic edge in sophisticated technological voter mobilization and targeting is eroding, as Republicans adopt similar techniques and catch up.
“Democrats have sold this myth about their magic on the ground,” said Brad Todd, a strategist for Mr. Gardner. “But they threw the best they had at us, and it wasn’t enough.”
Tuesday’s results are causing leaders of both parties, and those with their eye on the White House, to re-examine their assumptions about the electoral map.
While Republicans celebrated their showing in fast-growing swing states like Colorado, Democrats were deflated that their candidates did not perform better in Georgia and Texas, where they believed that demographic shifts, especially the growing ranks of Hispanic voters, were making the terrain more competitive.
Jason Carter and Michelle Nunn — the highly touted Georgia Democrats with famous last names who ran for governor and the Senate — both lost by eight percentage points, far worse than had been expected. The results were even more grim for Democrats in Texas, where the party poured tens of millions of dollars into an effort to make Wendy Davis’s bid for governor competitive and lay the groundwork for future advancement.
Ms. Davis was trounced by more than 20 percentage points and, notably, struggled to build up a big margin among Hispanics. Greg Abbott, her Republican opponent, aggressively pursued Latinos and highlighted his Hispanic mother-in-law during the campaign. He attracted 43 percent of Hispanic voters, according to exit polls.
2014 Midterm Elections Live Coverage
The Democratic coalition also depends heavily on women, but in many races, the party’s traditional advantage among female voters shrank or disappeared. Democrats were already debating on Wednesday whether the party’s message — which in some states, like Colorado, focused on reproductive rights — was adequate to motivate voters.
“Democrats spent an entire election cycle saying nothing to independents and left the center open to us,” Mr. Todd said.
Another question that Democratic strategists are grappling with in the aftermath of Tuesday’s vote is whether the so-called Obama voters — younger people, minorities and women — can be mobilized when the president is not at the top of the ticket. Many Democratic Senate candidates distanced themselves from the president, and it is unclear whether that discouraged his most ardent supporters.
Voters who turned out Tuesday were older, whiter and more conservative than those who participated in 2012. Sixty-five percent of those who voted were over age 45, compared with 54 percent in 2012. Seventy-five percent of the electorate was white; two years ago it was 72 percent.
One issue for Hillary Rodham Clinton, should she run for president, is whether continuing demographic changes making the country more diverse will be sufficient to ensure an electorate favorable to Democrats in 2016 and offset any decline in the excitement among minority voters inspired by Mr. Obama.
Interactive Graphic
The Most Detailed Maps You’ll See From Yesterday’s Senate Elections
Maps of precinct-level results from some of the closest Senate races.
“We shouldn’t just assume that the Obama voters will automatically come out for Democratic presidential candidates,” cautioned David Plouffe, Mr. Obama’s former campaign manager.
Longtime advisers to the Clintons were also digesting the implications of double-digit Democratic losses in places like Kentucky and Arkansas, where former President Bill Clinton’s base of white working-class voters has drifted from the party. Talk that Mrs. Clinton could compete in heavily white Southern states now seems likely to dissipate.
“For those voters who remain devoted to Bill Clinton, he’s not in office anymore, and they don’t like the dysfunction that’s up here,” said Paul Neaville, a Democratic strategist and Arkansas native who works in Washington.
If Tuesday was a sobering moment for Democrats, the results are likely to embolden a number of Republicans. Gov. Chris Christie of New Jersey, who headed the Republican Governors Association effort, may well interpret the success of his party’s candidates for governor in swing and liberal-leaning states as a positive sign about the appeal of blue-state executives.
Gov. Scott Walker of Wisconsin, having convincingly won a third race for the office in four years, could take his success in a Democratic-leaning Midwestern state as a positive cue. Former Gov. Jeb Bush of Florida would surely be pleased with how his party fared in places, like Colorado, that have rising Hispanic populations. And conservative hard-liners like Senator Ted Cruz of Texas could easily see the repudiation of Mr. Obama and repeated attacks during the campaign on the president’s health care law as a vindication of sorts.
Graphic
How Big Were Tuesday’s Republican Swings?
The Republicans took control of the Senate on Tuesday, picking up at least seven seats, and expanded their majority in the House. Their victory in the Senate was significant but not the largest historically — though it could rank among the top five election year swings since 1946.
“The field grew last night,” Senator Lindsey Graham, Republican of South Carolina, said in a telephone interview. “Everybody on our side will see what they want to see. And everybody will see some validation of who they are politically.”
But Mr. Graham, along with many center-right Republicans on Wednesday, suggested that the party should not take the success this year as a sign that they are bound to be competitive again in the most crucial states. Their problem is the opposite of Democrats’: When a diverse electorate shows up, Republicans struggle to win.
“There were a lot of people who didn’t vote last night and will in 2016,” Mr. Graham said. “Not to borrow from John Edwards, but there are two Americas — two American electorates. So I don’t think we should have a false sense of confidence from last night.”
One of Mr. Graham’s colleagues is already considering that larger presidential electorate. Senator Rand Paul, Republican of Kentucky, thinks his party needs to develop a broader and more inclusive appeal and compete in areas it now effectively cedes to Democrats.
He said in an interview that he would not decide on his own bid until next spring, but that in the meantime he planned to push measures that would help expand his and his party’s reach. He said he would lobby Republican leaders and Democrats to take up criminal justice reform and individual immigration measures when Congress reconvenes in January.
“When you look at presidential politics, when we’ve tried to go safe, we’ve been sorry,” Mr. Paul said, referring to both the party’s choice of candidates and where those hopefuls have campaigned. “We need to break out of the demographic stranglehold we’re in at this point.”