The Prosperous Justice Party shows its true colors.

Indonesia's Islamist political parties often try to put a gentle face on their extremist agendas. That charade has been unmasked by the largest such party's recent attack on a vice-presidential candidate's religious bona fides. And it shows the current leadership the risks of partnering with them.

The Prosperous Justice Party, or PKS, was part of President Susilo Bambang Yudhoyono's coalition in his first term and was the first to join his campaign for re-election last month. In return, Mr. Yudhoyono and his Democrat Party promised PKS a greater voice in the coalition, and control of the education ministry, a powerful post that would give the Islamists control over school curricula and a big budget.

How the party might use such influence almost immediately became clearer last week, when word leaked out that Mr. Yudhoyono was considering Boediono -- the current central bank governor and a career bureaucrat -- for his vice-presidential pick. PKS went into smear mode. Party president Tifatul Sembiring held a press conference and claimed that Boediono (who goes by one name) was unfit for office because he was an undevout Muslim and a "nationalist."

This is more than just a personal attack, and a false one as well. The implication is that Indonesia, a majority Muslim nation, must have Muslim leadership. That idea runs counter to the principles enshrined of Indonesia's 1945 constitution, which establishes a secular, law-based democracy that provides equal rights for all, regardless of faith.

Mr. Yudhoyono, to his credit, held firm and announced Boediono as his partner for the July presidential elections last week. Surprise, surprise, the PKS backtracked and fell into line. They must have sniffed the political winds: Mr. Yudhoyono holds near-70% approval ratings and is polling well ahead of the two other presidential contenders.

Yet one thing seems certain: Mr. Yudhoyono has given political parties like the PKS political credibility by accepting them into his coalition. This won't be the last time that they test his ability to stick to a secularist agenda.

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