(CNN) -- Voters in Texas began casting early voting ballots Monday in the bitter GOP Senate runoff election between Lt. Gov. David Dewhurst and his tea party-backed rival Ted Cruz.
An email sent to Dewhurst supporters encouraged them to get ballots in early, saying they could help upend a dysfunctional Washington establishment.
"We need David's Texas values in Washington to completely uproot the entrenched insiders and fight to implement the successful Texas model," the email read. "Vote early for David Dewhurst to avoid Election Day lines and unforeseen scheduling conflicts!"
The two Republicans have been dueling it out in an ugly race following the state's May 29 primary. Dewhurst failed at the time to win the 50% needed for the Republican nomination, resulting in a July 31 runoff vote between Dewhurst and Cruz, a former state soliciter general.
The race has evolved into one of the most expensive congressional contests of the 2012 election cycle. Through early May, Dewhurst spent more than $17 million to Cruz' $4.3 million, according to the most recent Federal Election Commission records available.
While Cruz lags behind Dewhurst in fund-raising, the tea party favorite has ample support from national conservative groups, including super PACs for FreedomWorks and Club for Growth, the latter of which has spent $1.5 million in the runoff alone.
In an election year with strong anti-Washington sentiment, Dewhurst has made Cruz' ties with the nation's capital a major attack line in the race
-- CNN's Ashley Killough contributed to this report.