BERLIN, June 18 (Reuters) - German Chancellor Angela Merkel"s ruling conservatives will promise "a decade of modernisation" on Monday in an election programme intended to see off the fading challenge of their Green Party rivals. The manifesto will counter the Greens" offer of a "new start" and show the conservatives closing ranks after a divisive battle over who should be their candidate to replace Merkel, who will step down as chancellor after a Sept. 26 federal election. Christian Democratic Union (CDU) leader Armin Laschet, who is now the frontrunner to become chancellor, hopes the programme will cement the conservatives" recently regained lead over the Greens in opinion polls and secure victory in September. "We want to make the "20s a decade of modernisation," said Silvia Breher, a deputy leader of the CDU, the senior partner in an alliance with the Bavarian Christian Social Union (CSU). "We want to make our country better and protect the liberties of present and future generations: with effective climate protection, secure jobs, sound finances and fairness in social issues," she told Reuters. Such policies are aimed at holding the political centre ground following the COVID-19 pandemic in Germany, Europe"s biggest economy, and winning over voters hesitant about entrusting the chancellery to the less experienced Greens. After enjoying strong popular support last year, the ruling "grand coalition" of the CDU/CSU and the left-leaning Social Democrats (SPD) has seen voters grow more frustrated with its handling of the pandemic. "Winning will not be enough for Laschet. CDU/CSU need to outperform the Greens by a significant margin to prevent them from forming an alternative coalition," said Carsten Nickel at Teneo, a political risk consultancy. The Greens surged ahead of the conservatives in late April after they picked Annalena Baerbock, 40, as their candidate to run for chancellor, with her promise of change capturing voters" imagination. read more But since then, a regional election setback, criticism over a Christmas bonus payment that Baerbock failed to declare to parliament and a suggestion that Germany should arm Ukraine have hurt the Greens. read more Two polls on Wednesday showed the conservatives seven and eight points respectively ahead of the Greens, whose leaders acknowledge they have been through a difficult period. COALITION CONUNDRUM In a debate on energy policy with Baerbock and other chancellor candidates on Wednesday, Laschet, 60, touted his own environmentalist credentials and played up his experience by recalling his role in implementing a phase-out from coal power. "The Greens have not governed in recent years and therefore have nothing to do with the exit from coal," he said. In a draft of the manifesto obtained by Reuters, the conservatives ruled out tax increases, aligning them with their preferred coalition partners, the business-friendly Free Democrats (FDP), and distinguishing them from their main rivals. read more The presentation of the manifesto is an opportunity for the CDU and CSU to show unity after Laschet beat Bavarian Markus Soeder in April in the race to be their parties" joint chancellor candidate. "The Union is always strong when it is united. Everyone involved is aware of that," said Breher. The latest polls would not give the CDU/CSU enough support to form a coalition with the FDP but point to just enough support for a CDU/CSU coalition with the Greens, or a Greens-led tie-up with the SPD and FDP. One Greens official expressed a clear preference for the latter scenario, asking: "If we can have the chancellery, why would we team up with the conservatives?".