Under a Donald Trump presidency, a Republican Congress and White House will be able to attempt sweeping rollbacks of President Obama’s landmark policies, including the health-care law known as Obamacare and last year’s nuclear deal struck with Iran. The Senate Democrats’ threat of a filibuster would, in many instances, be the only viable way to block such legislative efforts.

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Democrats had to pick up at least four seats if Hillary Clinton became president, and five under Trump, to seize the Senate majority. They never came close.

Democrats picked up only one seat, in Illinois, where current Rep. Tammy Duckworth unseated incumbent Republican Sen. Mark Kirk. Of the competitive races, it was the seat most likely to flip to Democratic control.

Democrats also won a competitive Senate race in Nevada, where former state attorney general Catherine Cortez Masto beat Republican Rep. Joseph J. Heck. But the seat, which is being vacated by retiring Senate Minority Leader Harry M. Reid, already belonged to Democrats.

Democrats maintained a shot early Wednesday at eking out a victory in New Hampshire, where Republican Sen. Kelly Ayotte and Democratic Gov. Maggie Hassan were running neck-and-neck with almost 90 percent of precincts reporting.

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But elsewhere, Democrats did not score the victories they had hoped for.

In Pennsylvania, Republican incumbent Sen. Patrick J. Toomey flouted almost all polling in the days before the election showing him closely losing to his Democratic challenger, Katie McGinty, while in Wisconsin, Republican Sen. Ron Johnson defeated former senator and favored Democratic candidate Russ Feingold to hold onto his seat.

North Carolina Republican Sen. Richard Burr prevailed after coming under a serious challenge from Democrat Deborah Ross, holding an important seat for the GOP.

Burr, the Senate’s top Republican on intelligence matters, including cybersecurity, has comfortably kept his seat for two terms. Democrats had hoped to oust him in a wave of swing-state victories to take back the Senate majority.

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Democrats also suffered losses in other Senate races they had sought to contest more closely during the campaign season.

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Arizona Republican Sen. John McCain cruised to an easy victory Tuesday night over his Democratic challenger, Rep. Ann Kirkpatrick, despite months of polls showing the two were neck-and-neck.

McCain had broken ahead of Kirkpatrick in the most recent polls, but the race was still considered to be among the more competitive ones in the country.

And in Indiana, Republican Rep. Todd C. Young beat Democrat Evan Bayh, who served two terms in the Senate and hails from one of Indiana’s most powerful political families. He was recruited as one of the Democrats’ stronger candidates but ultimately fell short against Young, a three-term congressman from the southern part of the state.

In Florida, Republican Marco Rubio, who ran an unsuccessful presidential campaign earlier this year, was also reelected to his Senate seat Tuesday night, keeping it in the GOP’s column as he overcame a challenge from Democrat Rep. Patrick Murphy.

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House races in Florida yielded mixed results.

Florida Democrat Stephanie Murphy staged an upset against one of the GOP’s veteran members, Republican John L. Mica, who has held his seat since 1993. Mica’s race in part was fueled by redistricting that led to a defeat for the normally safe incumbent, who ultimately lost by nearly three points.

Democrats also picked up a House seat in Florida from Republican Rep. David Jolly, who lost his reelection battle to former Florida governor Charlie Crist. Crist, who was a Republican and an independent while governor, joined the Democratic Party in 2012.

But Democrats lost the House seat being vacated by Patrick Murphy — who lost to Rubio in the Senate race — when Florida Republican Brian Mast defeated Democrat Randy Perkins.

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In Virginia, Republicans also overcame a Democratic effort to oust Rep. Barbara Comstock, who easily rode to a nine-point victory over Democratic challenger LuAnn Bennett.

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In general, House Democrats were frustrated and disappointed at less-than-expected gains.

Minority Leader Nancy Pelosi (D-Calif.) blamed FBI Director James B. Comey’s decision late last month to tell Congress about possible new information pertaining to the Clinton email probe. Many Democrats had charged that Comey’s decision was politically motivated, and Pelosi said Tuesday that it had a “definite impact” on Democrats’ efforts to seize more congressional seats.

In the House, it was possible that Democrats could knock out one of the GOP’s chief attack dogs against the Obama administration in California’s Darrell Issa, who as the former chairman of the House Committee on Oversight and Government Reform was banging the GOP’s Benghazi drum before there was a Benghazi committee. If Issa loses to Democrat Doug Applegate, a former Marine colonel whose camp has ridiculed Issa’s recent attempts to recast himself as a friend of the Obama administration, it could be the Democrats’ biggest House upset of the night.

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Some Democratic gains came in urban areas where Trump’s candidacy created negative headwinds: In Illinois’s 10th, Rep. Bob Dold (R) lost to Democratic challenger, former congressman Brad Schneider, while in New Jersey’s 5th District, Rep. Scott Garrett (R) lost to Democratic challenger Joshua Gottheimer. But in Iowa’s 1st District — another bellwether — Republican Rep. Rod Blum held out against Democratic challenger Monica Vernon.

Other competitive House races were all over the country, from New Hampshire to Nevada.

The race in New Hampshire’s 1st District was called after 4 a.m. Wednesday, with former Democratic congresswoman Carol Shea-Porter defeating incumbent Republican Frank Guinta.

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In Nevada’s 3rd District, Democrat Jacky Rosen defeated Republican Danny Tarkanian, while in the 4th District, Democrat Ruben Kihuen beat incumbent Republican Rep. Cresent Hardy.

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In general, Democrats performed better in Nevada, which boasts a large Latino voting population, than they did nationwide.

Early Wednesday morning returns were inconclusive in Minnesota’s 2nd District, where Democrats backing Angie Craig tried to link Republican talk radio host Jason Lewis to Trump over his comments about “not-thinking” women and “cultural suicide” by the “white population.” One of them will replace outgoing Rep. John Kline (R).

In the Senate, meanwhile, the parties held onto their seats in other high-profile races. Republican Sen. Rob Portman of Ohio, who just a few months ago was on the shortlist of incumbents most in danger of losing their seats, was reelected to a second term in one of the earliest races to be called Tuesday night. And in Iowa, Republican Sen. Charles E. Grassley won a seventh term over Democrat Patty Judge, despite earlier fears that his tenure was in jeopardy.

In Maryland, Democrat Chris Van Hollen was elected to his first term as a U.S. senator in the seat being vacated by retiring Sen. Barbara A. Mikulski (D), while in California, Democrat Kamala Harris was elected to her first term in the seat being vacated by retiring Sen. Barbara Boxer (D).

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Democratic Sen. Charles E. Schumer (N.Y.), who is expected to replace Reid as the Senate’s top Democrat next year, won a fourth term, while Washington Democrat Patty Murray, also a member of the leadership team, won her fifth term. The Senate’s No. 3 Republican, John Thune (S.D.), was reelected to his third.

In Georgia, Republican Sen. Johnny Isakson was reelected to a third term in a race against Democratic challenger Jim Barksdale. While Isakson was not in grave danger of losing his seat, it was not clear that he would pass an important 50 percent threshold to avoid a runoff until the last days of the election.

Elsewhere in the Senate, Vermont’s Democratic Sen. Patrick J. Leahy was reelected to his eighth term, while Alabama Republican Sen. Richard C. Shelby won his sixth, and Idaho Republican Mike Crapo won his fourth. Republicans Tim Scott of South Carolina and James Lankford of Oklahoma and Democrat Brian Schatz of Hawaii were reelected to their first full terms in the Senate, while Connecticut Democrat Richard Blumenthal and Republicans Rand Paul (Ky.), Mike Lee (Utah), Jerry Moran (Kan.), John Boozman (Ark.), and John Hoeven (N.D.) each won a second Senate term.

Colorado Democrat Michael F. Bennet was also reelected to his second full Senate term, while Oregon Democrat Ron Wyden was reelected to his fourth full term. Republican Sen. Lisa Murkowski of Alaska was reelected to her third full term, while the Senate race to replace outgoing Sen. David Vitter (R) in Louisiana will head to a December runoff between Republican John Kennedy and Democrat Foster Campbell. The outcome of that runoff, in which the Republican is favored, will not affect which party holds the majority in the Senate.