FACTBOX: Democrats building a longterm majority in Congress?

(Reuters) - If Democrats do well in Tuesday's congressional election, as is widely anticipated, it is conceivable that they could be on their way to building a long-term majority in the House of Representatives and Senate.

History shows that it is not unusual for one political party to dominate one or both chambers for long stretches of time.

Following are some examples:

* Democrats enjoyed a four-decade House reign that began in 1955, during Dwight Eisenhower's presidency, and ended in the beginning of 1995, midway through President Bill Clinton's two White House terms. Fiscal and ethics concerns helped House Republicans end the long Democratic run.

Democrats were nearly as successful in the Senate during this era, holding majority control in all but six years.

* Republicans then held the House for a dozen years, from 1995 through 2006, when they were booted from power amid, once again, fiscal responsibility and ethics concerns, along with the long war in Iraq.

* Republicans controlled the House from 1895-1911 and 1919-1931. But as the Depression years wore on, voters put Democrats in charge from 1933-1945, often by huge margins. Democrats also held Senate majorities from 1933-1947, hitting a zenith in 1937-1939, with 75 seats in the 100-member chamber.

* Incumbents have staying power. One reason political parties can maintain their holds on Congress for so long once they get them is that those in office generally have more money than their challengers to campaign for re-election. They also enjoy greater name recognition and "perks" of office, such as government funds for mass mailings to communicate with constituents.

* Shock waves. When one party wears out its welcome with voters, a "throw-the-bums-out" mentality can sweep the country. The Republican revolution of 1994 gave them 54 more seats in the House the following year.

In the post-Civil War era, Republicans gained a whopping 64 House seats in 1873. Their success came as the House, reflecting the country's growth, increased its membership to 293 seats, from 243 the previous legislative session.

* Conversely, power can sometimes be fleeting. Democrats should take note of the many times that majority control flipped from one party to the other after short stints.

Republicans took control of the House in 1881, 1947 and 1953, for example, only to lose it two years later each time. Senate Democrats faced the same fate in 1893. In 2001-2003, both parties squirmed through a 50-50 tie in the Senate, during which time leaders worked out a power-sharing deal, but Republican Vice President Dick Cheney, as president of the Senate, could break tie votes.

(Reporting by Richard Cowan; editing by Philip Barbara)