Reports continue to flow in of a large, combat-ready Russian force building up near Ukraine’s eastern border. That suggests Russian president Vladimir Putin is at least giving himself the option of a new invasion.

The question is what would prompt one. Although Putin’s movements in the Ukraine crisis have seemed unpredictable, one potential trigger is to stop Ukraine’s May 25 presidential elections and the consolidation of what he views as an antagonistic, pro-Western government on Russia’s border.

A little over a month after Ukrainian president Viktor Yanukovych was ousted and Russia invaded Crimea, Russia’s foreign ministry today denied that there is a threatening Russian force on the border; just a training exercise, it insisted.

But among the troubling signs that it’s not are that Russia has sent field hospitals as well as tens of thousands of troops, some of whom are taking steps to conceal their positions (paywall). They include motorized units, suggesting the capacity for a blitzkrieg. A US official yesterday said the Russian troops do not appear to be carrying out any exercises. In an afternoon note to clients today, Eurasia Group’s Alex Brideau put the chance of an invasion at 40%.

With Crimea dependent on Ukraine for its lights and heat, Putin’s objective may be to establish a land route directly from Russia to make sure its newest territory isn’t cut off. Analysts think he may therefore have in mind seizing the majority-Russian-speaking Ukrainian cities of Kharkiv, Luhansk and Donetsk.

The way Russia rationalizes its actions in Ukraine is that ethnic Russians and Russian interests have been and may still be in danger from Ukrainian nationalists. That the West dismisses this as a pretext is irrelevant. Reasons keep showing up for Putin to act. This week, for instance, a YouTube recording (in Russian) surfaced in which Ukrainian political leader Yulia Tymoshenko apparently threatens Putin personally. In the tapped phone conversation, Tymoshenko expresses the wish that she could “grab a machine gun and shoot that motherfucker in the head.”

The Tymoshenko remark—apparently intercepted by a spy agency, probably Russia’s—was recorded two days after a March 16 Crimean referendum validated the region’s shift to Russian control. In a phone call with a national security adviser, Tymoshenko says, “It’s about time we grab our guns and go kill those damn Russians together with their leader.” Tymoshenko asserts that parts of the call were doctored, but she did not disavow these passages.

At Foreign Policy, Russian military analyst Pavel Felgenhauer cites the May election as an inflection point that Putin may wish to disrupt. Tymoshenko, who was prime minister before being jailed by Yanukovych, this week announced that she would run for president in May. The next six weeks are a prime period for an invasion, Felgenhauer writes. “The Kremlin may find it hard to resist the temptation to attack Ukraine and ‘liberate’ the south and east while Russia is ready, the Ukrainian military weak, and the regime in Kiev unstable,” Felgenhauer says.

But in a blog post, New York University professor Mark Galeotti says time is actually running out for Putin. The best time for an invasion would have been last week. “Every week that goes by is a week for the Ukrainians to prepare both militarily and politically,” Galeotti told Quartz in an email exchange. (In his post, Galeotti provides a granular description of what a Russian invasion might look like once it starts. Here is Galeotti in an interesting podcast released today.)

Still, with the invasion of Crimea, Putin demonstrated his capacity to surprise. He could demonstrate it again, even if Ukrainian troops stand in the way.