Bad Luck and Missteps Make G.O.P."s Senate Climb Steeper WASHINGTON - The Indiana Senate candidate Richard E. Mourdock's reintroduction of rape and abortion into the political dialogue this week is the latest in a series of political missteps that have made the Republican quest to seize control of the Senate a steeper climb. Once viewed as likely to win the Senate, Republicans are now in jeopardy of losing seats in Massachusetts and Maine. If they do, they will need to win at least five seats held by Democrats and hold three other Republican seats at risk to net the three needed to take the Senate if Mitt Romney wins the presidency. If President Obama prevails, Republicans will have to win at least one additional seat in a state where they are seen as slightly behind - in Connecticut, Florida, Ohio or Pennsylvania. "Republicans can do it," said Jennifer Duffy, a Senate political analyst at the nonpartisan Cook Political Report. It's just getting a lot harder. Rob Jesmer, the executive director of the National Republican Senatorial Committee, said Thursday that Republican candidates were within reach of victory in 10 to 12 competitive races, with Mr. Romney's improvement in the polls lifting candidates in states that were out of play six weeks ago. But time is dwindling. The implications for the next two years cannot be overstated. If Mr. Obama wins a second term, his hand would be much strengthened by a Democratic-led Senate, even a narrowly divided one, as opposed to unified Republican majorities in the House and the Senate. At the moment, Democrats are given little chance of winning the net 25 seats they would need to take the House, though they could well gain House seats. If Mr. Romney wins the White House, even a one-seat Democratic majority in the Senate could thwart his domestic ambitions, from an immediate repeal of Mr. Obama's health care law to his proposed 20-percent across-the-board tax cut. Those proposals rely on both a Republican majority and the use of a parliamentary budget procedure that would negate a Democratic filibuster. But a Democratic majority leader could keep the proposals from coming to a vote without substantial Democratic changes. An opposition majority would also ensure leverage in the fight over the so-called fiscal cliff, what to do with the expiring Bush-era tax cuts and looming across-the-board spending cuts, called sequestration, that go into force in January. Senator Patty Murray of Washington, the chairwoman of the Democratic Senatorial Campaign Committee, predicted that Democrats would hold on. "If you look back two years ago, not one person thought we would be able to keep the majority," she said. Our prospects are very good. Republicans recognize that the climb back to the majority has become much harder than it appeared a year ago. Some of the difficulty was just bad luck. The unexpected retirement of Senator Olympia J. Snowe of Maine changed the math early. Some was self-inflicted. Mr. Mourdock's comment in a debate that when conception resulted from rape it was intended by God followed Representative Todd Akin's assertion that conception would not result from "legitimate rape." That comment turned a Missouri Senate race that Republicans expected to win into one where the Democratic incumbent, Claire McCaskill, could survive. The fierce battle between grass-roots conservatives and Washington power brokers that hurt Republican Senate ambitions in 2010 has also continued. Mr. Mourdock's primary victory in May over Senator Richard G. Lugar, a six-term veteran, had already put Indiana into play before his explosive comment. Expensive and divisive primary fights in Wisconsin and Arizona ended the way Washington Republicans wanted, with victories for former Gov. Tommy Thompson and Representative Jeff Flake. But both nominees emerged bruised and broke. Now, for the Senate's balance of power to switch, Republicans need a night of good fortune rivaling the Democratic sweep of 2006, when six Republican incumbents were defeated. Democrats currently hold 53 seats, but that number could rise if, as expected, former Gov. Angus King of Maine, an independent, wins the seat held by Ms. Snowe and caucuses with the Democrats. Elizabeth Warren, a Democrat, is also seen as having the advantage in her Massachusetts battle with Senator Scott P. Brown, a Republican. Republicans are clearly favored to win a Democratic seat in Nebraska. From there, they would have to win Democratic seats in Montana, North Dakota, Virginia and Wisconsin to reach 50, enough for the majority if Mr. Romney wins the presidency.