Larry J Sabato: This is what happens when you over-promise and under-deliver

With the Tea Party successes in the 2010 elections, Republicans are arguably more conservative than at any time since the 1950s, or perhaps even the 1920s

Americans have loved checks and balances since the beginning of the Republic. The idea is that power should be divided in so many ways that no one politician or institution can dominate.

Voters are on the verge of building in yet another check on Tuesday. Unless virtually every poll and pundit is wrong, the Republicans will score a substantial victory over President Obama's Democrats. The Grand Old Party (GOP) is expected to take the House of Representatives with many votes to spare, wiping out the 39-seat margin the Democrats built in the 2006 and 2008 elections.

While Republicans are unlikely to seize the Senate too, they will pick up six to eight seats – a sizeable gain for one election. The President's party will be left technically in charge, but by a narrow margin, and under the rules, small majorities do not freely rule.

The most significant gain for Republicans will be at the state level, where they will probably win most of the 37 contested governorships, including a raft of states in the North-east and Midwest that normally trend Democratic.

When the votes are counted, the GOP is likely to have well over 30 of the 50 state governors in their column, plus hundreds of additional state legislators and a dozen state legislative chambers – just in time for the critical redistricting that realigns all major political districts following the decennial census. Those who draw the lines can add sizeable numbers of their fellow partisans to state legislatures, and the House in Washington, in future elections.

Most midterm elections result in gains for the party not holding the White House. But the magnitude of the shift in 2010 is considerable, coming so close on the heels of the Democratic near-landslide of 2008.

Perhaps 75 per cent of today's swing can be attributed to the bad economy. Yes, President Obama inherited an economic disaster, but Americans are famously impatient and Mr Obama contributed to his own woes by over-promising and under-delivering. Swing voters are also unhappy with the national debt, now $14,000bn and climbing, and think Mr Obama's healthcare reform legislation – his signal achievement so far – will be too expensive and unworkable.

Under the US system, which requires agreement by both houses and the president before legislation can be passed, nothing much will happen without bipartisanship. And on that score, it is hard to be optimistic. With the Tea Party successes in the 2010 elections, Republicans are arguably more conservative than at any time since the 1950s, or perhaps even the 1920s.

In this election, Democrats are also losing most of their moderate ballast, and thus the remaining Democratic caucus in Congress will be more liberal than ever. Given the parties' dramatically different worldviews, it would be a struggle for them to agree on the wording of a Mother's Day resolution.

The Republicans may play right into President Obama's hands, however. Many of their office-holders and voters seem to view the 2010 election as an endorsement of their comprehensively conservative approach to governing. In fact, the election is a repudiation of the Obama Democrats, a corrective message-sending to the President, rather than an embrace of the GOP, which was just sent packing two years ago after the disappointing Bush era.

Soon, President Obama will have to prove his mettle in ways unnecessary while Democrats controlled large majorities in Congress. If he can do so, and if the economy finally begins to bounce back, he can still win a second term. Ironically, his role model will be Bill Clinton, who used a hard-right GOP Congress to position himself in the middle and win re-election in 1996. In his 2008 campaign, Mr Obama said he didn't want to be another Clinton – too temporising and moderate for an insurgent candidate's preferences. But times and the voters have changed, and Mr Obama's world will never be the same.

Professor Sabato is director of the Center for Politics, University of Virginia, and the author of 'Pendulum Swing', a book on the 2010 US elections