Tracking polls can be a little sketchy because they only track a continued snapshot of the same electorate over long periods of time.  Because a ‘tracking poll’ only measures a group of voters who are engaged early in the election process (in this example beginning December ’15) such polling methodology tends to miss the segment of the population that tunes-in later in the process, when it matters more.
As a consequence of methodology, tracking polls tend to miss the enthusiasm aspect which becomes more important as the race narrows and more people engage.  However, that said, the current NBC/Survey Monkey tracking poll shows a significant upward shift for Donald Trump and a downward trend for Ted Cruz and John Kasich:

Because of the methodology, the results are on the lower end of actual sentiment.  Meaning the national support for the upward shift is understated, and the drop in the downward trend is also understated.  In essence, Donald Trump is likely doing much better nationally than this representative snapshot, and Cruz/Kasich are likely doing much worse.

(Via NBC/Survey Monkey) […]  Support for Trump among most demographic groups has remained consistent in this week’s tracking poll compared to previous weeks. However, when just looking at Republicans, excluding independents who lean toward the Republican Party, he now enjoys 49 percent support compared to 43 percent last week. This 6-point gain is important, as Trump usually does well among independents, but has struggled to win over more traditional Republicans so far. Support for both John Kasich (15 percent) and Ted Cruz (28 percent) is down among Republicans compared to the past few weeks.  (read more)
To see the full poll data pdf – CLICK HERE