Opposition Republicans are almost certain to win next month’s congressional elections in the United States but their victory might be bittersweet. As long as President Barack Obama remains in the White House, there is probably not going to be a change in policy.

With the president’s approval rating close to his lowest ever in Gallup’s poll at 40 percent, members of his Democratic Party in otherwise conservative states are unlikely to hold on to or win seats in the majority of those that hold senatorial elections this year. Chances are Republicans will pick up seven more seats altogether, giving them a majority of two in the upper chamber.

Senators are elected for six year terms and a third of them are up for reelection every two years.

All member of the House of Representatives, by contrast, face election every two years. But despite Republicans’ unpopularity nationwide — Pew, another polling firm, puts their favorability at 37 percent compared to 46 percent for Democrats — their is even less chance of them losing control of the lower chamber.

Largely this is because districts, both Democrat and Republican, have become more ideologically homogenous in recent years and decades.

Earlier research by the Pew Research Center found that the share of Americans who express consistently “conservative” or “liberal” (meaning leftist) views has more than doubled since 1994, from 10 to 21 percent. Ideological overlap has diminished. 94 percent of Democrats are now to the left of the median Republican whereas 92 percent of Republicans are to the right of the median Democrat. Twenty years ago, the figures were 70 and 64 percent, respectively.

These strong views lead Americans to erect what Pew calls “ideological silos.” It is not just political life that has become more polarized; it is the whole of life. “Liberals and conservatives disagree over where they want to live, the kind of people they want to live around and even whom they would welcome into their families,” says the institute, making “blue” states even bluer and “red” ones even redder.

Most congressmen then fear facing their opposite number in the general election less than they do primary challengers who do not have to account for making compromises that legislating in a divided government after all entails. (To the extent that legislating is done at all these days.)

If Republicans take control of both chambers of Congress, they will still have a Democratic president to cope with for at least two more years. Given that Obama has the power to veto anything that comes out of the legislature, the election result is unlikely to produce a breakthrough in the gridlock that has held the two parties from tackling major issues such as entitlement reform, immigration and overhauling the United States’ overly complicated tax code.