The Senate race in Nevada between Catherine Cortez Masto, a Democrat, and Representative Joe Heck, a Republican, is gripping on several levels. Nevada is the only state where Republicans have a solid chance of winning a Democratic Senate seat this year. The seat has belonged for 30 years to Harry Reid, the current minority leader, who is not seeking re-election. A Republican victory would thus be not only a loss for the Democrats but a personal embarrassment for a venerable power broker. It would also be a triumph for the Koch brothers, who say they’re seeking poetic justice after years of being denounced by Mr. Reid for their conservative politics, and are spending lots of money to make that happen.

Mr. Reid calls Mr. Heck, a three-term congressman, “an absolute stooge for these right-wing nut cases.” He hand-picked Ms. Cortez Masto, a former Nevada attorney general, to succeed him and is throwing the state Democratic machine behind her battle in a race that is now too close to call. Ms. Cortez Masto, the granddaughter of a Mexican immigrant, would be the first Latina to enter the Senate. Her victory would be a proud repudiation of Donald Trump’s anti-immigrant platform in a state where Latinos’ proportion of the electorate has grown to 17 percent from 10 percent in 2004.

Ms. Cortez Masto addressed a host of problems during her years as attorney general, from 2007 to 2015: drug use, fraud schemes exploiting senior citizens, domestic violence and sexual assault.