By selecting Sen. Lloyd Bentsen of Texas as his running mate, Massachusetts Gov. Michael Dukakis has deprived the upcoming Democratic national convention of its last element of suspense.

But he has also moved to give the Democratic ticket geographical and ideological balance while compensating for his own lack of Washington experience.And he is boldly carrying the fight to George Bush on Bush's own home base - Texas, where Bentsen defeated Bush for the U.S. Senate in 1970.

The choice of the moderately conservative, multi-millionaire chairman of the Senate Finance Committee should make the Northeastern liberal Dukakis more competitive in the South and Southwest, where voters have been deserting the Democratic Party.

Yet, even if Bush still wins the South, Bentsen's selection could force Bush to spend more time in the South defending his natural political base and less time challenging Dukakis in California, Illinois, Michigan, and Ohio, key states in the battle for the White House.

Even so, the choice of Bentsen can't heal all the party's rifts. Certainly it won't please the Jesse Jackson camp, even apart from Jackson's own thwarted ambitions for office. As a senator, Bentsen has often supported the oil industry, business in general, and the more affluent - groups that liberals want to pay higher taxes. Likewise, Bentsen backed military aid to the Contra rebels in Nicaragua and won his Senate seat largely by opposing gun control.

If nothing else, the choice of Bentsen shows that Dukakis has a firm grasp on a major fact of political life - namely, that victory usually goes not to those whose appeal is restricted to the extremes of left or right but whose appeal is to the great mass of moderate Americans in the middle of the political spectrum.