Ted Cruz’s Senate re-election campaign has been sending voters in Texas a fund-raising letter in an envelope labeled “summons enclosed,” drawing criticism from some who called it misleading and raising questions about whether such a practice was legal. It is.

That is according to Myles Martin, a spokesman for the Federal Election Commission, who said the salient question was whether a mailing contains a disclaimer saying that it came from a political campaign. And this one did.

Aside from that, he said in an email, “the F.E.C.’s regulations don’t speak to how candidates may choose to word particular solicitations to potential contributors.”

Mr. Cruz is locked in an unexpectedly tight race against Representative Beto O’Rourke, a Democrat from El Paso, in a traditionally conservative state. Earlier this month, a top adviser to President Trump, Mick Mulvaney, the federal budget director, told a gathering of donors he believed it was possible Mr. Cruz might not win re-election because he is not “likable.”