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.”