Who’ll fill Kerry Senate seat? Question answered, but only for now.

Mass. Gov. Deval Patrick names a former aide, William 'Mo' Cowan, to fill the Kerry seat until a special election in June. Two House Democrats are girding for battle, as is the GOP's Scott Brown.

The governor of Massachusetts has appointed a former member of his staff to fill the US Senate seat that’s being vacated by John Kerry (D), but a longer term question looms: Who will voters choose just a few months from now to occupy that post?

It may boil down to a choice between three men, two Democrats and one moderate Republican, who are already familiar with the halls of Congress.

Mr. Kerry, a Vietnam veteran and former presidential candidate, is leaving the Senate to become secretary of State during President Obama’s second term in office.

For now, the Massachusetts seat will be held by William “Mo” Cowan, who stepped down in November as Gov. Deval Patrick’s chief of staff after previously serving as his legal counsel. In naming the interim senator, Governor Patrick followed a course he chose when another Massachusetts Democrat, Ted Kennedy, died in office: filling a Senate slot with someone who pledges not to run for permanent election.

That leaves the field open for what promises to be a lively contest, culminating in a June 25 vote.

A leading contender in the race has already emerged: Rep. Edward Markey, the state’s longest-serving House member in Washington. Known as a staunch liberal with a passion for issues related to energy and the environment, he has the backing of Kerry and many other top Democrats in the state.

But another long-time House member, with Boston roots, is poised to challenge him in a Democratic primary in April. Rep. Stephen Lynch, a former ironworkers union member, is expected to announce his candidacy Thursday. Representative Lynch is less liberal than Markey, criticized by some Democrats for his conservative stance on the sensitive issue of abortion.

Some other potential seekers of the seat, including Rep. Michael Capuano (D), have announced they won’t run.

On the Republican side, all eyes are on Scott Brown, the former state senator who won a surprise ticket to Washington three years ago in the special election after Senator Kennedy’s passing. In November, he lost that seat to Elizabeth Warren (D), and now he’s expected to launch a bid to return to the Senate.

Judging by the recent Brown-Warren race, in which the candidates were locked in a close duel for months while drawing in millions of dollars in contributions for advertising, the coming race could be another big one.

It’s bound to draw national attention in an “off” year, when the only congressional races will be special elections such as this one. As such, it could become a test of political winds amid partisan sparring on fiscal policy.

In that sense, Mr. Cowan’s role is important politically, as well as for the substantive votes he’ll be taking between now and mid-year.

Some battles over the future course of federal spending and taxes will come to a head this spring. Bay State voters may view the interim senator’s stance as emblematic of the Democratic Party in general – potentially a foil for Brown in his effort to appeal as a proponent of low taxes.

In the press conference at which his appointment was announced, Cowan was asked for his view on potential federal spending cuts that could cost jobs in Massachusetts and nationwide.

Cowan aligned himself generally with President Obama, saying the best approach to controlling federal deficits is a “balanced” one involving some spending cuts and some new tax revenue.

In a predominantly Democratic state, that view resonates with many voters. At least it did in November, when Ms. Warren ousted Brown.

Cowan said in the press conference on Beacon Hill that he would work hard during his tenure to “move forward the interests of this great state,” before returning to the private sector.

He will be the state’s second African-American to serve in the US Senate, the first being moderate Republican Edward Brooke in the 1960s and 70s.

Whoever wins the special election won’t have long to rest before their next campaign. The seat is up for a vote again in 2014.