Lt. Gov. David Dewhurst led former Texas Solicitor General Ted Cruz by 8 points in a recent internal poll obtained by Roll Call.

Fifty percent of respondents said they would vote for Dewhurst in the July 31 GOP runoff, while 42 percent supported Cruz. National Journal’s the Hotline first reported the results of the poll, which was conducted by Dewhurst pollster Mike Baselice.

It is a striking difference from a late June Cruz internal poll obtained last week that showed Cruz with a 9-point lead over Dewhurst.

The difference in the results appears to be rooted in how each campaign determines who is a “likely runoff voter.” The Dewhurst poll surveyed those who had voted in previous GOP primaries and stated an intent to vote in the runoff.

The Cruz poll surveyed only those who voted in the May 29 primary and stated an intent to vote in the runoff, according to Politico.

Open-seat races for Senate are rare in Texas, as are late summer runoffs. Determining exactly who will show up to vote is a difficult task. Every poll taken of this race is of limited value.

What the polls have in common is that Cruz has achieved a high name identification, which was a source of skepticism in his recent poll. According to the Dewhurst poll, 92 percent of those surveyed knew who Cruz was. In the Cruz poll, 96 percent were able to identify him.

Also in the new Dewhurst poll, 62 percent of participants said they had a favorable view of the lieutenant governor, while 24 percent viewed him unfavorably. Fifty-four percent of those surveyed had a favorable view of Cruz, while 24 percent viewed him unfavorably.

The poll was conducted among 605 likely runoff voters from July 5-8. The margin of error was 4 points.

In other Texas Senate news, Cruz is has reserved about $1.6 million in television ad space for the rest of the runoff election. That number could increase, and the conservative Club for Growth is also expected to supplement that ad buy.

It began this week in smaller markets and will expand to the Dallas-Fort Worth, Houston, San Antonio and Austin markets next week.

This is the race to replace retiring Sen. Kay Bailey Hutchison (R). Roll Call rates it as Safe Republican.