WASHINGTON — To understand the Republican imperative to capture the Senate in the midterm elections, look toward 2016, when a narrow victory this year could well be swept away.

The 2016 elections may be far off, but the Tea Party class of 2010 is shaping up to be problematic for the Republican Party. The challenges raise the real possibility that the party could seize control of both houses of Congress in 2014, only to lose it two years later as Democrats reclaim some of the seats lost in the Tea Party wave.

Even some Republicans say they are pressing that point.

“I’m helping win the majority in 2014, and I’m making the point that 2016’s going to be a very different map for us,” said Senator Rob Portman, Republican of Ohio, who will face his own headwinds running for re-election in a presidential year.

For Republican senators facing their first re-election in 2016, the climb keeps getting steeper.

In Wisconsin, Senator Ron Johnson faces accusations that he failed to report to the police allegations that a state lawmaker sexually assaulted one of his top aides. In Florida, Senator Marco Rubio has announced that he will not seek re-election if, as expected, he runs for president, opening a seat in a key presidential battleground.