Editor's note: Sen. Kelly Ayotte, R-New Hampshire, serves on the Senate Armed Services Committee. Rep. Peter Roskam, R-Illinois, represents that state's 6th District, serves as chief deputy whip and chairs the House Democracy Partnership. The opinions expressed in this commentary are solely those of the authors.
(CNN) -- On Sunday, Ukrainians will go to the polls to elect a new president three months after the ouster of corrupt former President Viktor Yanukovych.
We will be there in Ukraine to observe the elections as members of an International Republican Institute delegation, witnessing a vote that is an important step not only for the Ukrainian people's struggle for democracy, but for the entire region's hopes for long-term stability and democratic development.
In the lead-up to this critical vote, pro-Russian separatists, taking their cues from an increasingly aggressive Moscow, have attempted to undermine the elections in parts of eastern and southern Ukraine through threats and violence.
In Donetsk and Luhansk, separatist leaders announced they will not participate in the presidential elections, while heavily armed militia fighters seize control of election offices and government buildings.
Russian President Vladimir Putin has publicly called the election "a step in the right direction," but tens of thousands of Russian troops remain near the Ukrainian border, Russian intelligence forces continue operating in eastern Ukraine and Putin has been noncommittal about whether his government will recognize the outcome of the presidential vote.
As other countries in the region can attest, Russian interference in territorial sovereignty is nothing new. Since 1992, long before Putin came to power, Russia has occupied parts of Moldova, and for many years Moscow fostered a breakaway rebellion on Georgian soil, which eventually led to war in 2008.
Shortly before this year's Sochi Olympics, Russian forces used the pretext of security for the games to advance miles farther into Georgia, pledging to return to the ceasefire lines once the Olympics had ended. Instead, Russia's military has fortified its gains in Georgia and shows no indication of withdrawal.
Putin understands that successful elections and a new government committed to democratic ideals and anti-corruption will have a ripple effect throughout the region, including Russia.
In a recent interview, Georgian Defense Minister Irakli Alasania said, "If they [Ukrainians] survive this crisis there will be a future for all the countries in the region that border Russia." That is why Putin has undertaken extraordinary measures to undermine Kiev's effort to solidify its status as a sovereign, independent, and democratic nation.
To deter Putin's aggression, the United States must inflict immediate and serious economic consequences on Russia's financial, energy, and defense sectors. We must also expand and strengthen sanctions on Russian officials who were involved in the illegal annexation of Crimea. And Putin must know that additional acts of aggression will be met with even greater costs.
The vast majority of Ukrainians -- like the citizens of so many countries in the region -- envision their future as a united democracy, free from foreign intervention and intimidation, and integrated with the rest of Europe.
It is critical that Western nations support these aspirations through democratic assistance and economic development programs that support the new Ukrainian government as it enacts political and economic reforms to root out corruption, restore the rule of law and promote growth and prosperity for the Ukrainian people.
The United States and our European Union allies must not allow Putin to continue to meddle in Ukrainian affairs and use the specter of military aggression to undermine the safety and liberty of millions of people seeking democracy in the region.
It is our responsibility to stand with the Ukrainian people and their aspiration to live in a free and democratic society without fear of repression.