Five races tell the entire story of the midterms

Mr. Grijalva, 62 years old, is confident enough in the district’s leanings that he co-chairs the Congressional Progressive Caucus, a large group of liberal lawmakers. That, of course, makes him an especially inviting target for conservatives.

The Republican candidate, Ruth McClung, 28, is a physicist who works for a defense contractor. She’s never held public office before, and she’s embraced the tea-party movement.

Mr. Grijalva, by his own admission, has made one big strategic error: supporting a tourism boycott of his home state after it enacted a controversial immigration law. He’s since rescinded that position, but it’s proved damaging in the economically struggling area.

This race received little national attention until two weeks ago, when polls began showing it increasingly close. Some analysts now consider it a toss-up, while others see Mr. Grijalva as favored. The Democratic Congressional Campaign Committee, sensing danger, has aired ads attacking Ms. McClung.