On the Republican side, Lt. Gov. David Dewhurst and Ted Cruz, the former state solicitor general, agree on virtually every issue that could come before them in the Senate. But more than $40 million has been spent by the campaigns and outside groups trying to convince voters that the race provides an opportunity to upend all that is wrong with the federal government. If the wrong candidate wins, each side insists, the opportunity will have been wasted. “Ted is being viciously attacked by the establishment because he will bring real change to Washington,” former Gov. Sarah Palin of Alaska says in a robocall for Mr. Cruz. In Gov. Rick Perry’s latest advertisement for Mr. Dewhurst, he counters: “David’s the one candidate best prepared to make conservative change happen in Washington. Don’t let anyone tell you different.” The Democratic candidates, former State Representative Paul Sadler of Henderson and Grady Yarbrough, a former teacher, have significant differences on the economy and illegal immigration, but they are struggling to draw the attention of voters or donors, as the campaigns’ spending is well under $1 million. Texas has not elected a Democrat to statewide office since 1994. Mr. Sadler said that those who dismiss Texas as a “red state” are ignoring how far to the right Mr. Dewhurst and Mr. Cruz are. Democrats have the chance to present a viable alternative, he said. “The Republicans are singing the same hymnal,” Mr. Sadler said. “The real debate begins on Aug. 1, when there’s a contrast of substantive issues.” Mr. Yarbrough agreed that Democrats are being underestimated. He has spent much of his savings on broadcasting television advertisements aimed at black and Hispanic voters. “They’re the ones that put me in the runoff, and if I go to those voters and plead our case, I am sure they will come out again,” Mr. Yarbrough said. “I’m taking a $75,000 to $80,000 gamble here.” Although the two races are drawing significantly different levels of interest, both have turned on whether the voters should value legislative experience. Mr. Sadler says that only those who have held elected office are qualified to join the Senate. Mr. Dewhurst does not go quite as far but stresses his history of passing budgets and cutting taxes. “You could argue that there’s not that much difference between us, other than that I’ve done all the things Mr. Cruz says that he wants to do,” Mr. Dewhurst said. Both Mr. Cruz and Mr. Yarbrough dismiss “career politicians” as part of the problem in Washington. “All over the country, Americans are fed up with the same tired establishment incumbents that don’t believe in anything,” Mr. Cruz told voters in Willis this month. “There is a tidal wave sweeping this country as Americans are looking for new leaders who will stand and fight and get back to the Constitution.”