As the campaign to succeed President Barack Obama shifts into high gear, Republican voters seem assured of a contested race for their party’s nomination that will feature generational, geographical, occupational and ideological contrasts. On Monday, Sen. Marco Rubio of Florida announced his candidacy for the 2016 nomination, joining declared or almost-declared candidates Jeb Bush, the former Florida governor; Wisconsin Gov. Scott Walker; and Sens. Rand Paul of Kentucky and Ted Cruz of Texas.

All these candidates profess to be conservatives, but differences abound on issues from immigration to school reform to the U.S. role in the world.

There will be at least nine debates before the party’s convention next summer in Cleveland, offering ample opportunity for a clash on the issues and greater clarity about what the candidates believe and how they would translate their convictions into policy.

There also will be posturing and evasions, of course. But debates refine candidates’ positions in a way that inquisitions by the press cannot.

That brings us to the Democratic presidential field. In order to have a debate you have to have more than one candidate, and to have a debate that will command public attention you need a real contest.

So far the Democratic race consists of exactly one candidate with a truly national profile: former senator, secretary of State and First Lady Hillary Rodham Clinton.

Former U.S. Sen. Jim Webb of Virginia and former Maryland Gov. Martin O’Malley have indicated they might seek the nomination and have been courting Democrats in Iowa. But missing so far are two much better-known potential candidates: Sen. Elizabeth Warren of Massachusetts and Vice President Joe Biden.

In lamenting the lack of competition in the Democratic field, we aren’t suggesting that Clinton is an inferior or unqualified candidate. Far from it. Although she came to public attention as the wife of President Bill Clinton, her subsequent service as a senator and secretary of State mark her as an accomplished and entirely credible candidate. But her party and the electorate at large would be better off if she had to defend and define her candidacy in the crucible of a truly competitive race. And she might be a better candidate as a result.