Sen. Claire McCaskill (D-MO) (L) and Sen. Jim Webb (D-VA) hold a news conference to announce legislation to overhaul the federal government's planning, management, and oversight of wartime-support contracting at the U.S. Capitol March 1, 2012 in Washington, DC. The senators said they believe this is enough support in Congress to pass the legislation, which would build a greater system of accountability in military contracting. (Photo by Chip Somodevilla/Getty Images)

The controversy involving the Republican U.S. Senate candidate from Missouri, Rep. Todd Akin, is so critical not just because of his inflammatory remarks, but because control of the Senate could be determined by the winner of his race against incumbent Sen. Claire McCaskill.

Democrats currently control 53 of the Senate’s 100 seats, with 51 Democrats and 2 independents who usually vote with the Democrats. But they are widely expected to lose Senate races in North Dakota and Nebraska and have major challenges in Florida, Montana, Missouri and Wisconsin. If Republicans win four of those races, they will control the Senate.

That’s why Republicans are aggressively trying to dump Akin after his Sunday remarks that a woman who suffers from a “legitimate rape” is unlikely to become pregnant. They feel moderate voters will shun Akin if he remains in the race and back McCaskill, who polls show is very vulnerable in a state that leans Republican and voted for John McCain in 2008.

Control of the Senate is critical for a number of reasons. The House is widely expected to remain in control of the Republicans. So if Mitt Romney is elected, the GOP would effectively run all of Washington, potentially clearing the way for the party to pass some version of the Romney-Ryan economic plan, which includes major tax cuts for individuals, the vast majority of the benefits of which go to the wealthy, likely major cuts in spending on government programs in education, transportation and health care, and dramatic overhauls of Medicare and Medicaid. (Generally, legislation in the Senate requires 60 votes to pass, but budget and tax changes can be passed with just 51 votes through a parliamentary process known as reconciliation.)

If Obama wins, as current polls indicate is likely, a Republican Senate would still cause major challenges for him. A Supreme Court nominee by the president, for example, would need approval from a GOP Senate, raising the potential of Republicans blocking the appointment or forcing Obama to pick someone more conservative than Democrats would like.

A Republican House and Senate would look to repeal President Obama‘s health care law, and if he vetoed a repeal, they would attempt to cut funding from it.

Follow Perry Bacon Jr. on Twitter at @perrybaconjr