With the 2018 midterm elections about 18 months away, attention is shifting to the battle for the Senate — and who could emerge as potential challengers.

But history shows that prospective contenders have a few more months before they typically announce their candidacies.

Senate Republicans bear the brunt of the recruitment challenge this election cycle, since they are looking for challengers for 23 Democratic incumbents, and two independents who caucus with the Democrats. The GOP has an opportunity to expand its majority, since 10 of those Democrats are up for re-election in states that President Donald Trump won in November.

With some higher-profile Republican lawmakers passing on a Senate run, Democrats (and some GOP strategists, in recent media reports) say the party is struggling to recruit credible challengers. But Republicans counter that they still have time, and are in discussions with formidable recruits.

The timing of a candidate’s campaign announcement is just one of several factors in a campaign. And if recent history is any indication, Senate challengers would typically announce their campaigns between one and five months from now, or anytime from June to October.