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.
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.
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.
“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.