A new poll in South Carolina finds Donald Trump maintaining his large lead among likely Republican voters, but also shows Trump slipping slightly since Friday, while Sen. Marco Rubio and Gov. John Kasich edge higher.

The new poll was conducted on Sunday by the South Carolina House Republican caucus and follows their previous poll released Friday, ahead of Saturday’s Republican debate in Charleston.

But the two polls are actually very different internally. They have very different mixes of old and young, and they show rapid shifts in support among subgroups, making the two polls difficult to compare, and perhaps pushing their results far from reality.

In the new Monday poll, Trump leads the Republican field with 33 percent of the vote, down two points since Friday. Sen. Ted Cruz and Rubio are tied for second with 14 percent. Cruz is down two points since Friday and Rubio is up one point.

Former Gov. Jeb Bush is in 4th with 13 percent support, edging out Gov. John Kasich, who has 10 percent. Ben Carson is last, with just six percent support.

The overall results have changed little since Friday. Then, Trump led the field by 19 points. Today, Trump leads by 18 points. There has been considerable movement, however, in a few voter groups.

On Friday, Trump led among “conservative Republicans” by 15 points. Today, his lead among conservatives is down to eight points. On Friday, Cruz was the clear runner-up among conservatives. Today, he and Rubio are tied among these voters.

It should be noted that the poll released Monday contains many more “conservative Republicans” in its voting sample than Friday’s poll. Just over half the people interviewed in Monday’s poll described themselves as conservatives. In the poll released Friday, only 44 percent described themselves that way.

Monday’s poll also showed a wide swing among “Tea Party Republicans.” On Friday, Trump and Cruz split these voters. In Monday’s poll, however, Trump gained nine points and Cruz lost nine points among these voters.

There was a big swing among self-described “Libertarians” as well, with Trump losing 14 points among these voters and Cruz gaining 10 points. Marco Rubio and Ben Carson also registered big gains with Libertarians, but that was mostly due to the fact that Friday’s poll showed them with zero support among these voters.

Obviously, swings like these necessitate a word of caution. The key point to remember is that these ideological descriptions are self-reported. In other words, the person answering the poll describes his or her own political position from the range of options.

There is yet another caveat to this poll. In Monday’s poll, 64 percent of respondents were aged 62 or higher. In Friday’s poll, however, seniors 62 or older made up only 53 percent of the sample. That is a huge difference.

Seniors are by far Trump’s best age demographic. In both polls, Trump led his nearest rival by more than 20 points among Seniors. His margin over the other candidates tends to go down considerably as the voters get middle-aged. Trump’s margins grow again when voters are younger than 40.

An exact comparison with 2012 is difficult, because the age ranges used then are different than this poll. In 2012, however, only 27 percent of voters in the primary were 65 or older. Although the magnitude of the difference is uncertain, it is clear that Monday’s poll, with a 64 percent sample of seniors, is way too high.

It is likely this factor is inflating Trump’s lead, unless seniors end up voting in far higher proportions than they have in the past.

The main takeaway is that Trump still dominates the field in South Carolina, but voters seem to be moving around a bit. With a week of campaigning ahead, this movement may be a one-day blip, or the beginning of a trend.